1. Field of the Invention
The present invention relates to a tire monitor radio circuit for performing radio transmission to a tire pressure detector mounted on a wheel of a vehicle and a tire monitor system provided with the radio circuit.
2. Description of the Related Art
Tire monitor systems of the above-described type generally comprise a tire monitor device mounted on a body of a vehicle and tire pressure detectors mounted on respective tires of the vehicle. Information about inner pressure of each tire is transmitted by radio between the vehicle body and each wheel. JP-A-2005-119370 discloses one of such tire monitor systems. In one of known communication manners, a tire monitor device provided at the vehicle body side delivers a trigger signal by radio. In reply to the trigger signal, a tire pressure detector transmits results of detection of tire pressure by radio. In this case, the tire monitor device carries out an amplitude shift keying (ASK) modulation based on a digital baseband signal.
Furthermore, there is a time when a reception level of radio transmission from a first communication device is excessively high at a second communication device. In this case, for the purpose of improving reliability of radio communication accompanied by ASK modulation and demodulation, a technique is known which informs by radio the first communication device of the excessively high reception level so that transmission output of the first communication device is lowered. JP-A-2005-45451 discloses one of the above-described techniques.
The ASK modulation and demodulation result in the following problem. To modulate a digital baseband signal W10 as shown in FIG. 9, for example, a carrier wave is imparted to an antenna resonance circuit when the digital baseband signal W10 has turned to “1.” When the signal W10 has turned to “0,” impartment of carrier wave to the antenna resonance circuit is stopped. As a result, a modulated signal W11 shown in FIG. 9 is delivered from an antenna.
However, damped oscillation after stop of carrier wave impartment to the antenna resonance circuit is superimposed on the modulated signal W11. Accordingly, the modulated signal W11 contains an amplitude component S10 of damped oscillation and an amplitude component S11 corresponding to “1” of the digital baseband signal W10. When the signal is demodulated at the reception side, the amplitude component S10 is equated with the amplitude component S11, and a demodulated signal W12 is generated. Consequently, the demodulated signal W12 differs from the digital baseband signal W10, and accurate information cannot be transmitted. In the aforesaid conventional technique, however, a transmission output level is merely reduced. Accordingly, the difference between the amplitude component S11 corresponding to “1” of the digital baseband signal W10 and amplitude component S10 of damped oscillation cannot be rendered distinct. Moreover, since reduction in the transmission output renders the signal weaker against noise, accurate information still cannot be transmitted.